Breast cancer deaths are most commonly caused by the metastasis of tumor cells to distant organ sites that favor their growth. Bone is a frequent site of metastasis of some of the most common human malignancies including breast, prostate and lung cancer. Bone metastases are devastating to the patient as they are associated with intractable pain, hypercalcemia (abnormally high concentration of calcium in the blood), disabling pathologic fractures, and nerve compression syndromes. Statistically these skeletal-related events occur at the rate of 2-4 every 12 months25, and can be reduced by only about 50% with the use of the most effective drugs currently available for managing the host microenvironment, namely the bisphosphonate zoledronic acid (ZA) 26. This strongly suggests that better therapeutic approaches are required to more effectively treat patients with bone metastases. We propose that approaches focusing on early tumor growth and establishment in bone, rather than the later stages that bisphosphonates target will better inhibit tumor- induced bone disease. The late stages of bone disease have been previously well-described as a vicious cycle of bone destruction where TGF- (transforming growth factor-beta) is released from the bone stimulating tumor cell growth and the production of bone resorbing factors, which in turn releases more TGF- 27-32. Despite decades of research on these later stages, the early stages of cancer metastasis are not well-understood. However, it is clear that multiple factors and cell types within the tumor-microenvironment are important in these early stages of bone metastases. Our preliminary data and previously published data from other groups suggest that TGF-?is a major contributor to bone metastases and is likely important to these early stages. Based on our preliminary data, it appears that fibroblasts and the myeloid derived suppressor cells (immune cells present in the bone marrow and spleen) can stimulate tumor-induced bone disease, and that their function is regulated at least in part by TGF-beta. Thus, we have generated multiple mouse models in the immune-compromised Rag2 -/- mice with TGF-beta signaling ablated or constitutively activated in either the myeloid or mesenchymal lineage. This will allow us to investigate the importance of TGF-beta signaling in specific cell populations during the development of bone metastases. Furthermore we have obtained a TGF-beta inhibitory antibody for testing in pre-clinical models of breast cancer metastasis to bone. This proposal seeks to better understand the role of TGF-beta signaling in the tumor micro-environment and to develop strategies for treating bone metastases early. We hypothesize that TGF-beta signaling induces a permissive micro- environment for the establishment and growth of cancer cells in bone to initiate bone resorption. Blocking TGF-beta specifically at this early stage, before bone resorption, will inhibit tumor establishment in the bone and subsequent bone destruction. To address this hypothesis we propose to 1) Determine the effect of cell-specific alterations of TGF-beta signaling in the bone microenvironment on tumor establishment, growth, and bone disease. 2) Evaluate the effectiveness of early bone-specific TGF-beta inhibition on bone metastases, 3) Test the effectiveness of combination therapies that inhibit both TGF-beta with Gli2 on bone metastases.